"We are retiring the existing Messenger service globally ... and bringing the great features of Messenger and Skype together," Microsoft said, adding that Messenger will continue to be available in mainland China. Last November, Skype president Tony Bates confirmed in a blog post that Messenger would be shuttered in the first quarter of 2013.

The company also encouraged users to "update to Skype," and sign in using their Microsoft account (formerly known as a "Windows Live ID"), which includes an email address and password. All Messenger contacts will be included in Skype upon sign-in, the email said.

Those who are signed in to Messenger on desktop will see a banner notification to "upgrade," it added. When clicked, a pop-up window to install Skype will appear, and will automatically uninstall Messenger.

Until March 15, Messenger "will continue to work as you know it today," Microsoft said, but after that date, users won't be able to sign in, and will be prompted to switch to Skype.